A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
For certain applications, the area of substrate that is to be imaged for a single layer of a device may exceed the maximum cross sectional area of a projection beam of a lithographic apparatus. Such applications include large image sensors, such as charge-coupled devices (CCDs), large photonic devices, and thin film heads. A composite image can be formed from image portions projected onto target portions of the substrate that are smaller than the maximum cross sectional area of the projection beam. The image portions are separately provided by or to the patterning device and projected sequentially onto the surface of the substrate. For such applications, image portions projected onto adjacent target portions of the substrate must be closely aligned. For some applications, accurate alignment of adjacent image portions projected onto adjacent target portions of a substrate may be essential in order for the finished device to work. If two adjacent image portions are not correctly aligned within a predetermined tolerance band, it may only be detectable after the device has been completed and is subjected to electrical testing.
It is an aim of the present invention to obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere.